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On the whole I've had no major issues and have often used this forum when running into a problem and it's served me well. However I've come across an issue of quite severe horizontal ridges at regular intervals on parts printed using one of the machines and I am stumped - hence my first post.

You can see from the image below what the problem is.

The column should be straight and smooth like in this one, printed on another machine - same gcode, both white PLA.

I was suffering from underextrusion with a couple of my machines and after running through some of the remedies (which didn't solve the problem) I decided to try tightening the short belts - loosening the 4 fixing screws for the stepper motors, pressing down on the motor and re-tightening each screw. I did the same procedure on both machines and it's seemed to have eliminate the underextrusion but for one of them I'm now getting these ridges.

I printed a z-resonance test piece to see if it was anything to do with the geometry but the same thing happened and the ridges are at the same interval, approx. 3mm

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This problem happens to people occasionally and figuring it out is not so simple. First you should know it's probably one of two things:

1) Temperature related.

2) Z screw related

Did you also change the firmware? Because the PID controllers for both the bed and the nozzle have changed such that people get these kinds of banding caused by temperature variation. This problem for you is slow enough that it would have to be over a full minute which would be BED related.

Anyway the best test is to watch the temperature very carefully in the TUNE menu while printing this object and make sure the temperature isn't changing by even 2C on a regular basis.

I would also try printing the exact same above part but at 2X width and depth so that it spends more time on each layer - just to see if the pattern is now 6mm instead of 3mm. If it moves to 6mm this also indicates it is probably temperature related.

After you eliminate temperature that leaves Z screw issues. This is very likely the problem - especially since the threads on the z screw are 3mm apart. Z screw issues are much harder to find typically. I would start by turning off the power and pushing the bed up and down and feeling it and watching it. Also maybe turn the z screw by hand to see if you can feel an issue roughly once per rotation. There is a Z nut - slide the bed all the way up and inspect the 4 screws on the Z nut to make sure they are all tight. You might have to just order another Z screw and Z stepper and Z nut. The Z screw could be warped/bent. Or it might be rubbing against something causing it to move up and suddenly down on each rotation. It may be something very subtle.

You could also install pronterface and command the z screw to move slowly up and down and feel the bed to feel if something happens every 3mm. Pronterface/printrun is here (it's a very easy to use gui that gives you incredible control over your printer to test servos and temperature and calibrate stuff and so on):

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If the temperature goes up and down when the temperature is warmer you extrude more (even only 2 or 3C but typically 10C is needed for most filaments to see a difference) and you get a wider layer or horizontal ridge. When temp goes down you extrude a little less (filament slips very very slightly more) and you get a narrower horizontal layer.

If z lead screw is always commanded .1mm but instead moves .101mm (or .099) and then every 20th layer it slips a bit to get back to "proper position" then you get occasional thicker or thinner layers because the z screw moved more or less then it was supposed to and so the printer is extruding too much or too lilttle for that Z movement.

I checked the z- lead screw and and surrounding screws which were a bit loose so I tightened them slightly. There didn't appear to be any significant play in the lead screw nor anything interfering with it.

I printed another z-resonance test piece and monitored the temperature but there wasn't any fluctuation on either the bed or nozzle temp - both stayed around the set parameters only going up and down 1 degree occasionally. The ridges were still on the column, perhaps a slight improvement but no major difference.

I'll print a larger section as you suggest to see if the ridges are still present and if the interval changes at all.

Thanks again

Incidently I printed a couple of Robots using the settings you'd given in this thread